Currently, mobile communication networks are generally circuit-switched (CS) and include, for example, global system for mobile communications (GSM) and code division multiple access (CDMA). Operators have established comparatively perfect and abundant service platforms based on the CS network. Among them, a mobile switching center (MSC) is responsible for call routing and service logic execution such as call forwarding and call hold services.
The service provision of the CS network needs the support of MSC in a roaming area, which is disadvantageous to the introduction of new services. An IP multimedia sub-system (IMS) is a package-switched (PS) based service network and the service provision thereof is irrelevant to the roaming area, which is advantageous to the introduction of new multimedia services. The IMS supports a user equipment (UE) to access an IMS network via various PS access networks (for example, a general packet radio service (GPRS) access network) to carry out IMS multimedia services.
The PS-based IMS network is a development trend for communication networks in the future, but the IMS network is also a relatively complex network. It can be predicted that the full deployment of the PS-based IMS network may not be completed in a short time, and meanwhile, it is impossible for all users of the CS network to start using new IMS terminals in a short time, so that the CS network will coexist with the IMS network for a long time. Therefore, the CS network and the IMS network need to be operated at the same time.
In the evolution towards the IMS network, the 3rd Generation Partnership Project (3GPP) raises an issue of IMS centralized service (ICS), requiring to realize that a UE establishes an IMS call by bearing a voice or visual phone (VP) media on the CS network and meanwhile a service processing logic in the MSC is weakened or removed. The UE receives/transmits control signaling related to the IMS call by a newly introduced IMS CS control channel (ICCC). The ICCC may be borne by an unstructured supplementary service data (USSD) message and called ICCC-cs or borne by PS and called ICCC-ps.
An existing calling flow of establishing an ICS call via a CS network includes the following steps.
In Steps 101 to 104, a terminal equipment establishes an ICCC control channel with an IMS CS control function (ICCF) and sends the number of a called party to the ICCF via the ICCC control channel. Steps 101 to 104 are described in detail below.
In Step 101, the terminal equipment sends a session invitation (Invite) to the ICCF. The session invitation message includes the phone number of the called party, i.e., the number of a second party. The session invitation may be transmitted via a USSD or PS network.
It is understood that, the employed PS network may be a GPRS or any other packet access network, and the specific transmission mode may be selected according to network services subscribed by the user terminal.
In Step 102, the ICCF returns a provisional response message (183) that may include the number of the ICCF. The number of the ICCF may also be pre-configured at the terminal equipment, and thus does not need to be returned in this step.
In Step 103, the terminal equipment sends a provisional response acknowledgement (PRACK) message to the ICCF.
In Step 104, the ICCF returns a 200OK to acknowledge the provisional acknowledgement message.
In Step 105, the terminal equipment establishes a CS call leg with the ICCF.
In Step 106, the ICCF establishes a second call leg with the called party.
In Step 107, the ICCF establishes a call connection between the terminal equipment and the second party by binding the call leg established with the terminal equipment and the call leg established with the second party.
Here, the ICCF may be regarded as a back-to-back-user-agent (B2BUA) equivalent to two UEs bonded together. The first call leg established between the ICCF and the terminal equipment and the second call leg established between the ICCF and the called party are independent from each other, so that when one call leg is disconnected, the other call leg can exist independently. The establishments of the first call leg and the second call leg may be triggered by each other. During the establishment, the signaling exchange and media connection between the terminal equipment and the second party are realized, and the conversation between the terminal equipment and the second party is also realized.
A called flow of establishing an IMS call via a CS network in the ICS manner is similar to the above calling flow, and thus the details will not be described herein again.
The above technical scheme may establish the IMS call by the CS network, but during the conversation between the calling party and the called party or during the process of setting up the call, the circumstance that the calling party or the called party requires to change the media type occurs frequently. Though the IMS allows arbitrary deletion and addition of various media (voice, VP, text, etc.) in the process of a call, network equipments of most existing CS networks do not yet support the conversion of the media type after the call is set up. Therefore, in the above ICS scheme, after the user establishes a call connection with the IMS by CS, the user's experience of services is degraded due to the limitation of the current CS network that the media type of the call cannot be changed. Release5 (R5) defined by 3GPP enables the CS network to change the media type of the session during the session, but the existing CS network and equipments thereof need to be upgraded on a large scale, which results in a tremendous upgrading cost and is difficult to be deployed.